Nature of S-States in the Oxygen-Evolving Complex Resolved by High-Energy Resolution Fluorescence Detected X-ray Absorption Spectroscopy.

Photosystem II, the water splitting enzyme of photosynthesis, utilizes the energy of sunlight to drive the four-electron oxidation of water to dioxygen at the oxygen-evolving complex (OEC). The OEC harbors a MnCaO cluster that cycles through five oxidation states S ( = 0-4). The S state is the last metastable state before the O evolution.

Unusual P Hyperfine Strain Effects in a Conformationally Flexible Cu(II) Complex Revealed by Two-Dimensional Pulse EPR Spectroscopy.

Strain effects on and metal hyperfine coupling tensors, , are often manifested in Electron Paramagnetic Resonance (EPR) spectra of transition metal complexes, as a result of their intrinsic and/or solvent-mediated structural variations. Although distributions of these tensors are quite common and well understood in continuous-wave (cw) EPR spectroscopy, reported strain effects on ligand hyperfine coupling constants are rather scarce. Here we explore the case of a conformationally flexible Cu(II) complex, [Cu{PhP(O)NP(O)Ph-κ'}], bearing P atoms in its second coordination sphere and exhibiting two structurally distinct CuO coordination spheres, namely a square planar and a tetrahedrally distorted one, as revealed by X-ray crystallography.

Five-coordinate Mn intermediate in the activation of nature's water splitting cofactor.

Nature's water splitting cofactor passes through a series of catalytic intermediates (S-S) before O-O bond formation and O release. In the second last transition (S to S) cofactor oxidation is coupled to water molecule binding to Mn1. It is this activated, water-enriched all Mn form of the cofactor that goes on to form the O-O bond, after the next light-induced oxidation to S How cofactor activation proceeds remains an open question.

Water oxidation in photosystem II.

Biological water oxidation, performed by a single enzyme, photosystem II, is a central research topic not only in understanding the photosynthetic apparatus but also for the development of water splitting catalysts for technological applications. Great progress has been made in this endeavor following the report of a high-resolution X-ray crystallographic structure in 2011 resolving the cofactor site (Umena et al. in Nature 473:55-60, 2011), a tetra-manganese calcium complex.

Proton Translocation via Tautomerization of Asn298 During the S-S State Transition in the Oxygen-Evolving Complex of Photosystem II.

In biological water oxidation, a redox-active tyrosine residue (D1-Tyr161 or Y) mediates electron transfer between the MnCaO cluster of the oxygen-evolving complex and the charge-separation site of photosystem II (PSII), driving the cluster through progressively higher oxidation states S ( i = 0-4). In contrast to lower S-states (S, S), in higher S-states (S, S) of the MnCaO cluster, Y cannot be oxidized at cryogenic temperatures due to the accumulation of positive charge in the S → S transition. However, oxidation of Y by illumination of S at 77-190 K followed by rapid freezing and charge recombination between Y and the plastoquinone radical Q allows trapping of an S variant, the so-called S state (S), that is capable of forming Y at cryogenic temperature.

Structured near-infrared Magnetic Circular Dichroism spectra of the MnCaO cluster of PSII in T. vulcanus are dominated by Mn(IV) d-d 'spin-flip' transitions.

Photosystem II passes through four metastable S-states in catalysing light-driven water oxidation. Variable temperature variable field (VTVH) Magnetic Circular Dichroism (MCD) spectra in PSII of Thermosynochococcus (T.) vulcanus for each S-state are reported.

Can we trap the S(3)Y(Z)· metalloradical intermediate during the S-state transitions of Photosystem II? An EPR investigation.

We report the trapping of two metalloradical intermediates corresponding to the transitions S2 to S3 and S3 to S0 of the oxygen evolving complex (OEC) of Photosystem II (PSII), in preparations containing methanol, at temperatures near that of half inhibition of the respective S-state transitions. The first intermediate, with an EPR width of 160 G, is assigned to S2YZ, based on its similarity to the one previously characterized after trapping at 10 K. The second with a splitting of ∼80 G is tentatively assigned to S3YZ.

Conformational changes of the S2YZ* intermediate of the S2 to S3 transition in photosystem II.

The paper extends earlier studies on the S(2)Y(Z)* intermediate that is trapped by illumination in the temperature range 77 K to 190 K of untreated samples poised in the S(2)...

Conversion of the g=4.1 EPR signal to the multiline conformation during the S(2) to S(3) transition of the oxygen evolving complex of Photosystem II.

The oxygen evolving complex of Photosystem II undergoes four light-induced oxidation transitions, S(0)-S(1),...